Metrology for MEMS Tribology

Abstract

Microtribology is a new fast-growing field which combines a number of related disciplines, like physics of solids, physical chemistry and chemical physics, mechanics and electricity, etc. It is dealing with surfaces and surface properties (as versus bulk material properties), light loads and masses (as versus heavy loads) in application to micromechanical devices, as well as magnetic recording, surface/atomic force microscopy, surface microfinish, etc.

Theoretical and experimental microtribology data for the friction phenomenon on microscale is presented. Performance of numerous micromechanical devices is limited by wear and stiction. The model of wear combines adhesive, abrasion, and fatigue processes, as well as their dependencies on surface roughness and lubrication. The model of stiction includes processes of deformation, adhesion, and viscous flow, all related to perpendicular micro-displacements in the interface. The latter is critical for the modeling of friction-induced auto-oscillations, closely related to stiction. These models may serve as the basis for designing low-wear, stiction-free devices.

To verify the models and to study the friction, adhesion, and wear processes in depth, a new bench-top micro-tribometer mod. UMT has been designed. It has precision rotary and linear motions of both upper and lower specimens, with in-situ measurements of friction force and coefficient, adhesion, wear and micro-wear, acoustic emission and electrical resistance of contact. All the force, acoustics, and electrical sensors and amplifiers are of the original design, which ensures the high accuracy and low drift. It can work in pin-on-disc, ball-on-disc, flat-on-flat, 3-ball, 4-ball, screw-in-nut, and many other modes of operation, reproducing all types of lubrication conditions, from dry and boundary friction up to hydrodynamic lubrication. This apparatus can accommodate a wide variety of sample sizes (from a 1-mm ball up to an 8-inch wafer), sample shapes (round, rectangular, triangle, etc.) and materials (metals, ceramics, plastics, paper, etc.). Its variable speed can be as low as micron per second, which is critical for the studies of stiction and stick-slip mechanisms. It has fully automated motor-control and data-acquisition.

In this work, new results have been obtained on the micro-tribometer with greases, solid lubricants and unlubricated contacts. They have confirmed the theoretical models and have established the basis for building a database of microtribology data for MEMS.